1. Field of Invention
This invention concerns novel cermet materials comprising ceramic and metal components and a molten metal infiltration method and process for fabrication thereof. The cermets of the invention are fabricated from preformed ceramic powder infiltrated with a molten metal or metal alloy. The novel cermets are useful for a wide range of applications such as manufacture of surgical instruments, cutting tools, engine parts, wear parts, etc. Their properties, such as strength, durability, toughness, elasticity, etc. can be designed according to their intended use.
Additionally, the invention concerns cermets, particularly alumina-titanium cermets, having properties similar to bone. These cermets are made of components biocompatible with the human body and are suitable for bone and joint replacements.
2. Background Art and Related Disclosures
While the material technology has substantially advanced in recent years, providing many choices of materials having specific properties, there is still a need for materials having selected properties specifically suitable for specific purposes. These materials typically need to be durable, hard to break, non-fragile, non-brittle and yet elastic and reasonably light in weight. Additionally, their fabrication should be economically feasible and not overly laborious. Moreover, in order to meet requirements for their specific use, many of these materials need to be able to be custom designed.
Thus, it would be advantageous to have available a material having all above named properties and a process for fabrication of any such material where these properties could be easily varied and changed depending on used components and process conditions and design.
One example of the needed material having specific properties is the material having weight, structure, strength and other properties similar to and resembling bones or joints, which material would be suitable for bone or joints replacement.
Medical advances of past several decades have substantially extended life of the human population. The aging population, however, faces a multiplicity of disorders which may limit its quality of life. Among those disorders are osteoporosis, Paget's disease of bone and joints, and arthritis. All these disorders may cause limited mobility and often, particularly in the elderly, can result in death due to resulting bone fractures. When not fatal, these disorders still often require surgical bone or joint replacement of hips, knees, elbows, etc.
The major problem associated with the bone replacement is a lack of a suitable material which would have the same or similar properties as bone but that would also be compatible with the human body. The properties which the bone or joint replacement material need to possess include light weight, porosity, strength, durability, elasticity and, in order to prevent wear in joint areas and to prevent or allow tissue attachment in other areas, as need be, a possibility to be surface finished. Therefore, such material must have approximately the same porosity, weight and structure and must not be more fragile or more brittle than the normal bone.
Currently, several materials are known and medically acceptable as implants. While these materials, namely alumina ceramic (Al2O3) and titanium alloys containing 5% titanium and 4% aluminum (Ti5-4) or 6% titanium and 4% aluminum (Ti6-4), are acceptable as implant materials, alone or in combination, neither has the desired properties to replace bone or joints.
Thus, it would be advantageous to provide a biocompatible material which would have the strength, durability, elasticity and surface finish similar to the natural bone and which could also be custom shaped in a relatively short time so that surgeons could make necessary adjustments to the implant during the operation in order to properly fit the patient.
Ceramic/metal combination materials, known as cermets, are known. These cermets possess useful properties, such as toughness and strength, and have been used for manufacturing lightweight personnel armor, structural materials, cutting tools, radiation resistant structures, insulation materials, impact, abrasive and wear resistant structures, etc. However, none of the known cermets possess properties which would make them suitable for bone or joint replacement or for manufacture of other products requiring the similar properties as bone implants. This is due to the fact that the known processes for their preparation do not prevent ceramic powder particles shrinking, enlargement or cluster formation during their fabrication. These changes in particle sizes of the ceramic powder result in an uneven and unpredictable porosity of the resulting material.
A method for forming metal-filled ceramics is described in U.S. Pat. No. 3,718,441. Cermets which are boron-carbide-aluminum or boron-carbide-reactive metal composites are described in U.S. Pat. No. 4,605,440 and infiltration processing of boron carbide, boron and boride-reactive metal cermets is described in U.S. Pat. No. 4,718,941. Cermets prepared by combustion synthesis and metal infiltration are described in the U.S. Pat. No. 4,988,645. However, none of the above produced cermets possess specific properties as described above required for bone replacement or manufacture of other products. This is primarily due to methods and/or materials and conditions used for their fabrication.
There are two other methods currently known and used for fabricating cermets. The first method involves cold press and sintering (CPS). The second method involves hot pressing (HP) or hot isostatic pressing (HIP).
Cermets, such as cermets prepared from titanium-aluminum oxide (Ti-Al2O3) components, were prepared by the conventional process of cold pressing and sintering of the blended titanium (Ti) and aluminum oxide (Al2O3) powders. Typically, the blended titanium-aluminum oxide powders are formed into the desired shape, and submitted to a temperature at least as high as the sintering temperature of the titanium-aluminum oxide blend. This leads to a large shrinkage of more than 15% of the aluminum oxide particles. Additionally, these shrunk particle result in a grain growth and cluster formation of aluminum oxide particles occurs. The high sintering temperature to which these blends are submitted results in formation of a cermet containing dense aluminum oxide areas unevenly distributed throughout the matrix cermet interspaced with titanium filling-in voids between these unevenly distributed areas. This is due to the aluminum oxide particles sintering together into grains and clusters when submitted to the high sintering temperatures allowing the titanium migration only into the void sites between the growing grains of aluminum oxide clusters.
The growth of aluminum oxide particles into grains and clusters, therefore, does not allow an even distribution of the molten titanium within the sintered ceramic particles but rather results in molten titanium getting into voids between aluminum oxide grains and forming larger metal areas.
The cold pressing and sintering method thus results in cermets consisting of ceramic grains and clusters larger than 80 microns interspaced with unevenly distributed titanium areas larger than the titanium powder particle size used in the starting powder blend. Titanium thus forms distinct titanium islands within grains and clusters of the aluminum oxide matrix. Because of this uneven distribution due to the coarse dispersion of large titanium areas in a coarse structure of aluminum oxide grains, the mechanical properties of the formed cermets are poor and typically these cermets are fragile, brittle and their porosity and weight is uneven.
The cermets produced by cold sintering are not suitable for preparation of cermet materials which require that the material is light, tough, durable non-fragile or non-brittle and has a uniform porosity. These cermets do not have even distribution of titanium within the aluminum oxide matrix and are, therefore, fragile and subject to easy fracture. The shrinkage and grain growth occurring during the cold pressing and sintering are clearly not acceptable for bone replacement implant material which need to have a consistently porous microstructure strengthened with metal.
Some improvement on this microstructure was achieved by development of two subsequent methods utilizing the pressure during the sintering, namely hot pressing (HP) or isostatic hot pressing (HIP). This improvement consist of sintering of compositions of titanium in aluminum oxide (5 to 60 vol/%) at lower temperatures (1400-1600° C.) and by applying pressure to the powder preform while at these lower temperatures to force entry of titanium in between the aluminum oxide grains.
Hot pressing of the titanium-aluminum oxide powder blend is accomplished generally in a graphite die and punch assembly where the pressure is applied to the powder inside the dies through hydraulic force on the punches. The powders are heated to the desired densification temperature (1400-1600° C.) and the applied pressure assists in the rapid (<1 hour) densification of the powder.
Hot isostatic pressing is achieved by sealing the powder blend in a metal, such as, for example, molybdenum, or in a glass container and applying a gas pressure, generally argon or helium, to the outside of this container while heating the gas to the desired sintering temperature (1300-1500° C.).
The cermet products obtained by hot pressing or hot isostatic pressing have similar properties. Unfortunately, both these methods still result in the shrinkage and in the grain growth of the aluminum oxide-titanium powder blend and in an uneven distribution of the metal through the aluminum oxide. This is due to the same sintering problems observed during cold pressing and sintering, where the sintering is performed at lower temperatures and the process takes a longer time. Hot pressing or hot isostatic pressing take shorter time as they are performed under high pressure. Thus, while finer microstructures than those obtained during the cold pressing and sintering were obtained from hot pressing or hot isostatic pressing, such processing did not result in cermets having properties required for bone replacement. Even under relatively high pressure (<30,000 psi) conditions, the sintering process resulted in the growth of the titanium and aluminum oxide grains larger than 60 microns and in large shrinkage to achieve densification needed for bone replacement implant.
Therefore, the hot pressing improvement of the cold pressing and sintering process still does not provide material having a uniform distribution of metal throughout the ceramic matrix suitable for bone replacement or for manufacture of other products having similar requirements for material properties.
In an attempt to reduce the grain growth, special processing of the ceramic metal blend was suggested using coating the aluminum oxide particles with titanium metal. This was expected to allow the HP or HIP processes to achieve densification of the powder at lower temperatures and reduce grain growth. However, in order to be able to be coated with sufficient titanium metal and still achieve a dense product during hot pressing or hot isostatic pressing, the aluminum oxide particles must be larger than 40 microns. This limit prevents fabrication of cermets which would have no shrinkage, and would have uniform distribution of the metal within the aluminum oxide matrix assuring the strength, toughness and non-fragility of the bone replacement implant.
Thus it would be very advantageous to have available a material which would posses the above listed undesirable property and a method and process for its preparation eliminating the above listed disadvantages and problems.
It is therefore a primary objective of this invention to provide a cermet of which properties can be designed to specifically meet the requirements for its intended use. Due to the improved processing, the new cermet is light, has an even porosity, is strong, durable, elastic, and tough as well as non-fragile or non-brittle. The new cermet can be prefabricated into a near-net ceramic shape of the article to be used, and molten-metal-infiltrated after the final shaping. Additionally, the surface of the article can be surface finished in such a way that it fits its use. The new cermet which can be made of components fully compatible with the human body has properties similar to bone and is able to withstand the pressures and weight to which the bone in the body is constantly submitted without breaking. Additionally, it can be made to fit the patient bones and joints and surface finished to meet physiological functions of the replacement, such as tissue attachment, lubrication, etc.
All patents, patent applications, and publications cited herein are hereby incorporated by reference.